Thermionic power amplifier for delivering high power to low impedance loads



Dec. 23, 1952 B. M. HADFIELD THERMIONIC POWER AMPLIFIER FOR DELIVERING HIGH-POWER TO LOW IMPEDANCE LOAQS 2 SHEETS-SHEET 1 Filed Oct. 29 1945 RESISTANCE (waits) M m n W 4 INVENTOR BERTRAM MORTON HADFIELD ATTORNEY Dec. 23, 1952 M. HADFIELD B. THERMIONIC POWER AMPLIFIER FOR DELIVERING HIGH-POWER TO LOW IMPEDANCE LOADS Filed Oct. 29, 1945 2 SHEETSSHEET 2 "1 NVENTOR BERTRAM MORTON HADFIELD ATTORNEY Patented Dec. 2 3, 1952 THERM ONI POWER AMPLEFIER FOR DE- LI VERING HIGH rowan T Lovv IMPED- AN CE LOADS Bertram Morton Hadfield, Harrow Weald, England,. a ssignor to Automatic Electric Laboratories Inc., Chicago, 111., a corporation of Dela- Application October 29, 1945, Serial no, one In' Gr-eatBritain December 15, 1944 vide a circuitarran'gem'ent' employing a pentode' or beam tetro'd'e' valve" of conventionaldesign which is operated at a given steady 'anode voltage or from a steady anode oltagesupply', the

arrangement being such' tha't a power output greater than normal is obtainedinan anode load substantially lower than normal.

A further object-onthe invention is to provide a circuit arrangement employing a pent-ode or beam tetrode' valve asa power output valvewith an anode load' impedance-having.a value which is small compared with-the value necessary in a conventional circuit; v r g V According to one feature of the invention, in a thermionic valve circuit emplo-yed for feeding power into anoutput load and-- including one or more thermionic" valves of; the pentode or beam tetrodetype, having at least -on'e additional grid electrode between the control grid and the anode, the steady, positive voltage applied between the additional electrode and the cathode; is at least twice that of the steady,'.-positivevoltage applied betweenthe anode and cathode.

According to a furtherifeature o-i the invention, the ratio of the -steady,upositi ve voltageapplied between the-additional; electrode and the cathode to the steady, positive voltage applied between the anode and the cathode has a value which is greater than-unityand-which is determined by the value ofgthe-anode loadimpedance. In modern communication engineering-,. the

use of higher carrier frequencies-overtransmission media of increasingattenuation-andof lower characteristic impedance; requires that the amplifiers or repeaters deliver: high-energy levels into low impedances. While higherenergy levels can be obtained by-using: higher amplifiersupply voltages and lower; impedances can be matched to the output stage by higher ratiooutput transformers in theory, neither of these solutions is satisfactory; in practice because the normal high current anode-supply voltagemay. beapproximately 130 volts-and thepoorer high'frequencyfi response of high ratio Atransformers limits the overall performance of the system:: Furthermore the applicationwof voltage and: current negative feedback tothe amplifier to stabilise its gain and internal impedance at desired values becomes more diincult, owing to -the increased residuals of the feedback networkif the latter is made of high impedance to avoid the wastage of a substantial fraction of the available output. Again the anode capacitance of the output stage, which is considerably larger than that of the previous stages, will the more adversely affect the overall frequency characteristics if higher anode load impedances are used, while the'unwanted feedback from anode to grid in the' output stage, due to anode-gridcapacitance, i the more eiiective as the gain of the stage increases! In order to meet these requirements, it has been proposed to employ thermionic valves specially designed to pass high currents at low anode supply voltages. The application of the invention to such a communication systemhowever, enables pentode or beam tetrodes of conventional design to be employed, the only condition being that a low-current voltage supply is made available for the screen of a higher value than that available for the anode.

The invention will be better understood from the following description taken in conjunction with the accompanying drawings in which:

Fig. 1 shows some curves used to illustrate the principles of the invention while Fig. 2 shows, by wa of example, a circuit rangeme'nt embodying the invention.

The general designtheory for the operation of thermionic valves under-"conditions such that the limiting values of voltage and current are always just attained has been discussed in the Wireless Engineer, Vol.21, August 1944, pages 368 to 376. It is shown that, assuming the minimum anode current II) is negligible compared to the maximum anode current la, the anode output volt-amperes is given by and the anode volt-ampere efficiency is given by where p z: the ratio of the anode load voltage to the anode-cathode D. C. voltage drop (V) across the valve.

c a constant depending on the input Wave form.

N the ratio of the voltage excursions from given operating point, and

M theratioofthe' current excursions from the same operating point.

3. Also it can be shown that the anode load resistance is given by (VbVa) 1 V (N+1) (3) (Ia-Ib) Ia N where Vb and Va are respectively the maximum and minimum anode voltages.

It can also be shown that for a pentode or beam tetrode valve, the anode current is related to the screen voltage E in the following manner:

where a the grid-screen amplification factor Kaza constant depending on the type of valve Eg=a given control grid voltage and m=a positive number.

Further, if E9 is written as s.Es

where s is a positive number if positive, maximum grid-cathode voltage is used and vice versa, and Es -screen-cathode voltage, (4) now becomes It should be noted that this definition of the maximum grid-cathode voltage is in terms of the negative grid-cathode voltage which will produce substantially zero anode current. For normal non-grid current operation, 3 is zero and for grid current operation s is positive. The value assigned to s will depend on the nature of the input waveform and the desired bias condition. For instance for a waveform having equal positive and negative amplitudes and for grid current operation with zero bias, 8 is +1.

Further it can be shown that the greatest output for a pentode or beam tetrode (other things being equal) is obtained when the minimum anode voltage swing Va is made to coincide with the knee of the anode characteristic, and assuming this to occur at q times th screen voltage Es, then (1-p)V:q.Es

Hence Equation 5 above becomes,

Substitution of Equation 7 above in Equations 1 and 3 above gives the following,

Anode output volt-amperes:

WELL m m .5! m N (1+ v+ (8) peak voltage swing ratio (N), waveform (k), and since the value of q is substantially the same for all pentode or beam tetrode valves. then the maximum anode output is obtained when p(1-p) is a maximum, i. e. by diiferentiation, when Taking m as 1.5 as being a well accepted figure over the normal range of anode currents, then 17:0.4 gives the maximum anode output. Taking q as 0.2 (from inspection of characteristics of normal pentode and beam tetrode valves), it will be seen from Equation 6 above that the normal value of p which is used when Es:V, is 0.8. From Equations 6, 8 and 9 above it can now be seen that (other factors remaining constant),

(a) The anode output is 2.63 times as great if p0.4 instead of 0.8,

(b) The anode load resistance is 0.096 times the normal value, and

(c) The p value of 0.4 means that Before proceeding further to demonstrate the effects of using p values around 0.4, it is pointed out that for a resistance anode load (i. e. no choke or transformer) a maximum anode output for a given anode busbar voltage (Vb) and for all other factors constant except p, is also obtained when Es is three times Vb. This can be found by substituting in Equation 8 above, differentiating the resulting 0 function for a maximum, and substituting the p value obtained in Equation 6 above. This case is, however, of rare occurrence in practice, since clearly the maximum output will be smaller than for the choke or transformer coupled load.

Taking the choke or transformer coupled load case as being the more profitable (i. e. where the steady anode-cathode voltage V is substantially equal to the anode busbar voltage), the effects of using p values around 0.4 will now be described in order that the application of the invention may be appreciated.

In order to put the-description in terms of practical figures, the following will be assumed. Operation to a sinusoidal input is usual and gives N and M equal to unity, while It has the value 0.125; it is assumed that the output is also sinusoidal by the use of sufiicient negative feedback to the prior amplifying stages. The real assumption at this stage is that M and N are 1; the waveform of the output is immaterial to this discussion and the assumed value of It must be altered in accordance with theactual waveform; the assumption that k is 0.125 for a sinusoidal input is no more erroneous than the assumption that the corresponding output is also sinusoidal. As is normal, grid current operation will not be assumed, so that s is 0, while as stated before a value for q of 0.2 will be taken as representative of normal pentode or beam tetrode valves. The exponent m will be taken as 1.5, and the steady anode-cathode voltage as volts. The latter assumption permits operation from the normal volt supply with enough allowance for the steady voltage drop on the anode choke or transformer, and for automatic grid bias if required. The value of the valve factor 7 of 0.32. The grid bias willbe approximately onehalf the negative grid base of the valves, i. e.

and if ,u. be assumed to be 10. then the grid bias will be 17 volts; in practice this value should be varied in situ until equal incipient overloading on both positive and negative peaks of the input waveform is obtained, because the bias should be such as to give a steady anode current one-half the maximum; otherwise the calculated output of .10 watts will not be given.

Fig. 2 shows a circuit diagram of this amplifier, with the basic values of the output stage shown in figures. The input is, applied from terminals A, B, to the input transformer Tl of assumed step-up ratio 5:1, the secondary voltage being applied via a grid resistance R! to the grid of the first stage valve VI and ground. The valve VI is a pentode or beam tetrode having an anode impedance comprised by resistance R5 and inductance Ll in order to obtain a maximum of response at high frequencies, and the alternating voltage thereon is fed to the output stage via condenser Cl and resistance R8, over a grid resistance R9 to the grid and via the grid bias potential of approximately 17 volts to the cathode. The four paralleled output valves V2, V3, V4 and V5, have their screen grids commoned and taken to +340 volts, their cathodes commoned and taken to ground, and their anodes commoned via individual oscillation-stopper resistances RIEI, RI I, R12 and Rl3, of the order of ohms each, to the anode choke L2 designed to carry the steady anode current of 625 mA., and the small alternating component due to its inductance. The output is taken from the anode and cathode via the large condenser C2 to the output transformer T2 of unity ratio and the feedback network and on via the secondary winding of T2 to the load resistance L of 50 ohms. The series current feedback resistance R3 of 2.5

ohms is connected in series with the primary of 4 T2, while the shunt feedback ratio of 0.05 is provided by resistances R2 and RI. As the sum of the voltages on R2 and R3 must be fed into the cathode circuit of VI for negative feedback, R2 is made of convenient value to provide the necessary steady grid bias voltage for VI, say 300 ohms, when RI becomes 5,700 ohms. The loss on the shut feedback path is therefore less than one-hundredth of the wanted output, say 0.1 watt and is negligible. In order to prevent local feedback on VI, the screen is decoupled to its cathode by condenser C3 and a series anode resistance R6, which must be approximately 3000 ohms in order not to reduce unduly the value of R2 and R3. The positive busbar supply for the amplifier, other than that required for the output stage screens, has been shown at the conventional value of 130 volts, and allows 30 volts steady drop on the anode choke L2, i. e. a D. C. resistance of 48 ohms.

The provision of the higher voltage, low current, screen voltage supply for the output stage necessary for carrying out the invention may be made from the existing battery supplies by well known means such as a rotary converter, or a'vibrator unit, operated from the 130 volt or 24 volt batteries. Such means are readily available since they are required normally for the operation of radio receivers which use the conventional types of valves at around screen volt- 8 ages of 300 volts, where a main supply is not available; or alternatively, not reliable.

The invention has been confirmed experimentally by applying a steady anode-cathode voltage of to a valve such as the 6L6G, and three successive screen-cathode voltages of 100, 200 and 300. At each of the screen voltages the maximum resistance anode load was found by inspection of the output waveform, that resistance which just gave perceptible flattening of the positive and negative anode voltage swings being the correct one for a grid input just attaining anode current out off and grid current. The three anode load resistances resulting were found to be in the ratio of 1 :0.25:0.1 and occurred with peak voltage swings 0.8, 0.6 and 0.4 of the steady anode-cathode voltage, for screen voltages in the ratios of 100:2002300 respectively. The invention has also been used in an application where the operation of low resistance anode loads at high energy levels is a well known difiiculty. This occurs in the design of test oscillators covering a very wide frequency range. In this application it was required to generate an output of 6 watts in an anode resistance load of 600 ohms over a frequency range of 60 cycles/sec. to 1.5 megacycles/sec.; a range which practically precluded the use of an output transformer but enabled the use of a choke. By using steady anodecathode and screen-cathode voltages of and 300 respectively, these requirements were satisfied with a single valve of the KT66 type.

I claim:

1. In a power amplifier comprising a vacuum tube, a cathode, a control grid, 9. screen grid and a plate in said tube, means in said tube for suppressing secondary electron emission between said plate and said screen grid, a source of direct current plate potential and a plate impedance connected in series between said plate and said cathode for applying a positive direct current potential to said plate, an output load circuit connected between said plate and said cathode, the impedance of said load circuit being substantially less than the impedance conventionally used with said tube for delivering maximum power output at plate potentials and screen grid potentials that are approximately equal, means for applying an input signal potential between said control grid and said cathode to produce a signal current in said output circuit, a source of direct current screen grid potential, means for applying said screen grid potential to said screen grid, the screen grid potential being substantially greater than the plate potential, the tube having positive internal impedance at all times, the ratio of the screen grid potential to the plate potential being determined by the value of the load circuit impedance so as to cause maximum power to be delivered thereto, said output circuit having an impedance as low as approximately one-tenth of said conventional output impedance.

2. In a power amplifier as claimed in claim 1, a similar tube in parallel with said first tube, the anode, cathode, screen grid and control grid of said similar tube commoned to the anode, cathode, screen grid and control grid, respectively, of said first tube.

3. The combination of a power amplifier as claimed in claim 1 with a voltage amplifier, a tube in said voltage amplifier having at least a cathode, a control grid, and a plate, the control grid of said power amplifier coupled to the plate of said voltage amplifier, means for applying positive potential to the plate of said voltage will be taken as 20,00 since it, is found that a wide variety of normal valves give this order, for instance the following series: K'I66, EL50, 6L 6G nd RCA80'7 With these assumptions we have,

Anode output watts=14.p(;1--p) Anode load resistance, R 358. ohms (11) and Fig. 1 shows curves for these two quantities plotted on a logarithmic scale against values of 2 from 0 to l on a linear scale. By so plotting, the efiects of varying the assumed value of any of the constants may be seen at a glance, by shifting the curves bodily by an amount corre sponding to the ratio of the variation. For instance, reduction of the valve factor about twice the anode watts are obtainable for a range of 10:1 in anode load resistance, and the geometric mean of this resistance range is at least one-tenth the normal value with of unity (i. e. a p value of 0.8). It will also be seen that the order of resistance approaches more nearly that required for modern transmission media. For example, an output of 10 watts may be required for carrier operation on a submarine cable of impedance 50 ohms. This could be met with four valves. of the present type (or two having i Ka of 10,000) by operating them at a p value of 0.32 (i. e. with Es=3.4V) since the single valve output and resistance are 2.5 watts and 200 ohmsrespectively, and with an output transformer ratio of unity.

In general it will be seen that a wide range of anode load resistance is available for substantially the same output, by means of the invention, in that a proper choice of the ratio will provide the means for meeting any specific case.

rom the general Equations 8 and 9 above, while it is possible considerably to raise th output by increasing V above the assumed value of 100 volts, it is not possible to produce a large reduction in R, since the former is proportional to the 2.5 power of V and. the latter to the 0.5 power of V. Moreover there is a large choice of output pentodes or beam tetrodes whose maximum permissible screen voltage lies between 300 and 400 volts all .of which can beused with the invention if a steady anode voltage of is adhered to.

The invention also ameliorates the position as regards the provision of voltage and current feedback circuits without unduly afiecting the output or circuit design. In the case of the voltage feedback ratio, clearly the same resistance potentiometer across. the anode load will have about one-tenth the effect at ratios around three, since the anode load resistance required is one-tenth; that is, the watts lost in this potentiometer will be one-tenth. Alternatively if the normal potentiometer gives difficulty with residuals because of its high impedance, the latter can be reduced Without much additional loss. As regards, the current feedback resistance, difliculty is often experienced where anode chokes are not desired in that it must then be placed in series with the anode transformer primary winding and occasions a marked loss in steady anodev voltage. Now the value of the current feedback resistance is proportional to the anode load resistance, for a given voltage feedback ratio and a given ratio of load to internal impedance. From Equation 7 above, the steady anode current will be proportional to (lp)" (since the steady current is proportional to the maximum), while from Equation 9 above the anode load resistance (and hence the current feedback resistance) is proportional to p/ (1-p) Hence the product of these is proportional to 10, so that reduction of p value will reduce the steady voltage drop on the current feedback resistance in direct proportion. The incorporation of the current feedback resistance in the output stage without the use of a choke is therefore facilitated by the invention.

By way of example, and in order to show the application of the invention, the basic design of the output stage of the amplifier previously cited will now be undertaken. Assuming that a stabilised gain of 34 db is required, and as the output transformer ratio may be unity if four valves in parallel are used at a p value of 0.32, then 14 db can be obtained on the input transformer of ratio 1:5 step-up. This leaves 20 db to be obtained from the amplifier, and the use of one stage prior to the output will enable sufiicient open-circuit gain to be obtained, so that gain stability will be high and the feedback network may be calculated from the usual simplified formulae. On this basis the voltage feedback ratio required will be or 0.05 (the 2 is for the case of an internal impedance equal to the load), while the current feedback resistance will be 0.05 times the anode load of 50 ohms, i. e. 2.5 ohms. The steady anode current of the four paralleled output valves will be one-half the maximum- Ia, and can be calculated from Equation '7' above using a p value of 0.32, V=100 volts, q=0.2, s=0,

and m:l.5 as before. A value of 625 mA. is obtained. The steady screen voltage required will be 340, from Equation 6v above and. a p. value 9 amplifier from said source of plate potential, means for applying the negative potential of said last source to the grid of said voltage amplifier, a feed-back circuit including three resistors connected in series, the first and second ones of said resistors connected in parallel with said output circuit, the third one of said resistors connected between said output circuit and the negative pole of said plate potential source, the cathode of said voltage amplifier connected to the junction point of said first and second resistors, the resistors of said feed-back circuit of such resistance that the normal grid-to-cathode directcurrent voltage bias for the tube of said voltage amplifier and the required feed-back signal voltage are developed between said last cathode and the negative pole of said source of plate potential.

BERTRAM MORTON HADFIELD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,072,945 Farnham Mar. 9, 1937 2,266,541 Foster et a1. Dec. 16, 1941 2,386,892 Hadfield Oct. 16, 1945 

